1. Field of the Invention
The present invention relates generally to micro-electromechanical (MEM) devices for wavelength tunable lasers, and more particularly pertains to micro-electromechanical devices for wavelength tunable lasers which control the laser wavelength to match a wavelength selective device such as an external filter as required for a given application by using a dither wavelength locked feedback loop connected to the MEM device.
2. Discussion of the Prior Art
In recent years, micro-electromechanical devices or MEMs have matured into a viable technology for both electrical and optical communication systems. In particular, MEM devices can be used to build lasers with tunable wavelengths, also known as frequency agile lasers. This is especially important for dense wavelength division multiplexing (DWDM) systems which typically require from 32 to 64 wavelengths today and may require hundreds of wavelengths in the future. Today, each wavelength must be generated by a different laser, which in turn means a large number of part numbers and cards which must be tracked in ordering and manufacturing systems, or held in inventory as field replacement spares. Furthermore, cards must be swapped in order to reconfigure a network, such as a meshed ring network, with wavelength reuse.
The explanations herein discuss both wavelength and frequency, which have a reciprocal relationship (xcex=c/f, where c=speed of light), as is well known in the field of optics.
Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) are light-wave application technologies that enable multiple wavelengths (colors of light) to be paralleled into the same optical fiber with each wavelength potentially assigned its own data diagnostics. Currently, WDM and DWDM products combine many different data links over a single pair of optical fibers by re-modulating the data onto a set of lasers, which are tuned to a very specific wavelength (within 0.8 nm tolerance, following industry standards). On current products, up to 32 wavelengths of light can be combined over a single fiber link with more wavelengths contemplated for future applications. The wavelengths are combined by passing light through a series of thin film interference filters, which consist of multi-layer coatings on a glass substrate, pigtailed with optical fibers. The filters combine multiple wavelengths into a single fiber path, and also separate them again at the far end of the multiplexed link. Filters may also be used at intermediate points to add or drop wavelength channels from the optical network.
Ideally, a WDM laser would produce a very narrow linewidth spectrum consisting of only a single wavelength, and an ideal filter would have a square bandpass characteristic of about 0.4 nm width, for example, in the frequency domain. In practice, however, every laser has a finite spectral width, which is a Gaussian spread about 1 to 3 nm wide, for example, and all real filters have a Gaussian bandpass function. It is therefore desirable to align the laser center wavelength with the center of the filter passband to facilitate the reduction of crosstalk between wavelengths, since the spacing between WDM wavelengths are so narrow. In commercial systems used today, however, it is very difficult to perform this alignmentxe2x80x94lasers and filters are made by different companies, and it is both difficult and expensive to craft precision tuned optical components. As a result, the systems in use today are far from optimal; optical losses in a WDM filter can be as high as 4 db due to misalignment with the laser center wavelength (the laser""s optical power is lost if it cannot pass through the filter). This has a serious impact on optical link budgets and supported distances, especially since many filters must be cascaded together in series (up to 8 filters in current designs, possibly more in the future). If every filter was operating at its worst case condition (worst loss), it would not be possible to build a practical system. Furthermore, the laser center wavelengths drift with voltage, temperature, and aging over their lifetime, and the filter characteristics may also change with temperature and age. The laser center wavelength and filter bandwidth may also be polarization dependent. This problem places a fundamental limit on the design of future WDM networking systems.
A second, related problem results from the fact that direct current modulation of data onto a semiconductor laser diode causes two effects, which may induce rapid shifts in the center wavelength of the laser immediately after the onset of the laser pulse. These are (1) frequency chirp and (2) relaxation oscillations. Both effects are more pronounced at higher laser output powers and drive voltages, or at higher modulation bit rates. Not only can these effects cause laser center wavelengths to change rapidly and unpredictably, they also cause a broadening of the laser linewidth, which can be a source of loss when interacting with optical filters or may cause optical crosstalk. Avoiding these two effects requires either non-standard, expensive lasers, external modulators (which are lossy and add cost), or driving the laser at less than its maximum power capacity (which reduces the link budget and distance). Lowering the data modulation rate may also help, but is often not an option in multi-gigabit laser links.
It would thus be highly desirable to provide a stable, optimal alignment between a laser center wavelength and the center of a Gaussian bandpass filter in order to optimize power transmission through such fiber optic systems and reduce optical crosstalk interference in optical networks.
Accordingly, it is a primary object of the present invention to provide micro-electromechanical (MEM) devices for wavelength tunable lasers which utilize a dither wavelength locked feedback loop which compensates for many internal variables of the laser, a wavelength selective device such as filter, and the MEM with a single mechanism that stabilizes the laser and locks the wavelength to any desired value.
The present invention concerns wavelength selective devices which encompass wavelength selective devices of all types including filters of all types including comb filters, etalon filters and rotatable disc filters and wavelength selective gratings of all types including Bragg gratings and array waveguide gratings.
It is an object of the present invention to provide a servo-control xe2x80x9cwavelength-locked loopxe2x80x9d circuit that enables real time mutual alignment of an electromagnetic signal having a peaked spectrum function including a center wavelength and a wavelength selective device implementing a peaked passband function including a center wavelength, in a system employing electromagnetic waves.
It is another object of the present invention to provide a servo-control system and methodology for WDM and DWDM systems and applications that is designed to optimize power through multi-gigabit laser/optic systems.
It is a further object of the present invention to provide a wavelength-locked loop for an optical system that enables real time alignment and tracking of any spectral device that selects a wavelength, such as a Bragg grating, in optical fibers and waveguides, etc., for use in WDM systems.
It is yet another object of the present invention to provide a servo/feedback loop for an optical system, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that enables real time alignment of a laser with variable optical attenuators by offsetting an optical filter from a known transmission in optical fibers and waveguides, etc.
It is yet a further object of the present invention to provide a servo/feedback loop for an optical system, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that may be used in light polarization applications.
It is still another object of the present invention to provide a servo/feedback loop for an optical system, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that enables real time alignment and tracking of laser center wavelengths and filter passband center wavelengths in multi-gigabit laser/optical systems such that the optical loss of a WDM filter/laser combination is greatly reduced, thereby enabling significantly larger link budgets and longer supported distances.
It is yet still another object of the present invention to provide a servo/feedback loop for an optical system, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that enables real time alignment and tracking of laser center wavelengths and filter passband center wavelengths in multi-gigabit laser/optical systems such that lower cost lasers and filters may be used providing a significant cost reduction in the WDM equipment.
When employed in laser/optical networks, the system and method of the present invention may be used to tune laser diode devices, and/or compensate for any type of wavelength-selective element in the network, including wavelength selective filters, attenuators, and switches, in fiber Bragg gratings, ring resonators in optical amplifiers, external modulators such as acousto-optic tunable filters, or array waveguide gratings. This applies to many other optical components in the network as well (for example, optical amplifiers that may act as filters when operating in the nonlinear regime). Furthermore, the system and method of the invention may be used to implement less expensive devices for all of the above application areas.
Alternately, the system and method of the invention may be implemented to tune such devices for WDM and optical network applications, in real-time, during manufacture, e.g., tuning all lasers to a specific wavelength. This would significantly increase lot yields of laser devices which otherwise may be discarded as not meeting wavelength specifications as a result of manufacture process variations, for example.
The wavelength locked loop of the present invention enables a tighter control of wavelength, which allows an increased density of wavelength channels with less cross talk between channels in a wavelength multiplex system, which might typically include 32 or 64 channels or links. Pursuant to the present invention, each channel includes a separate wavelength locked loop which includes a separate source such as a laser and wavelength selective device such as a filter. Accordingly a wavelength multiplex system can include an array of 32 or 64 lasers and an array of 32 or 64 filters.